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Will gas "last longer" with the cap off?

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Dear Tom and Ray:



When my husband and I first got married (18 years ago), I noticed that when his car was low on gas, he would remove the gas cap, saying it would make the gas last longer until he could get to a station. I am a college-educated woman, and this seemed preposterous to me ... but then I noticed other people doing it. My husband has since stopped doing this, due to my nagging. But is there any truth to this, or is it just some urban legend? -- Tina

RAY: When the power goes out at your house, does your husband also run outside and disconnect the electric line from the pole so he can suck more electricity through the house's wires and catch the end of the ballgame?

TOM: He's nuts, Tina. I imagine his thinking (or his father's or grandfather's thinking, more likely) is that getting fuel from the gas tank to the engine is like pouring soda out of a two-liter bottle. And while it glug-glug-glugs out, if you were to punch a hole in the bottom of the bottle with an ice pick, you'd allow more air in, and the soda would pour out more quickly.

RAY: But the gas tank is not a soda bottle. First of all, nothing "pours" out of the gas tank. The gasoline is "pushed" out by an electric pump that sits right at the bottom of the tank.

TOM: And second, the pressure inside the tank is carefully managed by the fuel-tank ventilation system. That's done so that gasoline vapors don't constantly waft out into the atmosphere and make everyplace on Earth look like Los Angeles.

RAY: In fact, if you drive with your gas cap off, or even loose, your Check Engine light will eventually come on. The computer will conclude that the fuel system can't hold pressure, and will warn you that you need to have the car serviced.

TOM: We make a lot of money on that at the garage. Some guy comes in with his Check Engine light on, we walk around the car, tighten up his gas cap and stick out our hand for some money.

RAY: Actually, it's unethical for us to charge him for that, and we don't. But while he's there, no one says we can't sell him shocks and tires!

TOM: In the old days, gas caps had pinholes in them and gas was sucked out of the tank by mechanical fuel pumps driven by the engine. So that's probably where this theory originated. Even then, I'm dubious that removing the gas cap would have made any difference. But nowadays, it's absolute bull feathers, Tina. And you were right to hound him until he stopped doing it.
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